The Efficient Home Forum

I am wondering whether someone can give me a bit of detail about when the boiler switches off.

If just a single room is calling for heat, will the radiator heat the room enough to then stop heating and be able to switch the boiler off, or will it hover at about the requested temperature, keeping the boiler running?

This is important because it is a problem I have been having with another smart heating system, Heat Genius: the system will keep the boiler on, and the radiator on a tiny bit, so that it dribbles heat into the room, to try to keep it at exactly the requested temperature, but the result is that the radiators without TRVs stay on constantly, the boiler never switches off, and rooms without TRVs become unnecessarily hot.

Answers much appreciated.

With TPI, as the document posted above explains, cycling the boiler on and off frequently (10 minute cycles by default) when there is only a small but non zero demand is by design. The proportion of time that it is on for each 10 minute cycle controls the total amount of heat put out by the boiler.

This is to keep the temperature as steady as possible rather than having the room temperature cycle up and down at least +/- 1 degree, this steady temperature is much better for comfort and means you can typically set your room temperature about 1 degree lower than an old fashioned thermostat since the temperature is not dipping below the set temperature for long periods of time.

I know on our old mechanical wall stat we had before Evohome that it had at least a 2 degree differential band - so if it was set to 20 it wouldn't turn off until at least 21, and once it did turn off it wouldn't come back on again until the hallway dropped below 19 degrees. The problem with that is that the boiler would go off for as much as an hour and a half to two hours at a time since the hallway cools so slowly, meanwhile other rooms like the living room would drop 2-3 degrees in that time as they were unable to get any heat. So we'd have to go "tweak" the hallway thermostat when we noticed the living room was chilly, which after a while resulted in the hallway being a lot hotter than the living room. Not good. Not comfortable and not efficient.

Although Evohome also solves this problem by letting each room call for heat for itself rather than relying on a central thermostat (the living room is able to command the boiler on even if the hallway isn't cold) if we had simply replaced the hallway thermostat with a modern digital TPI thermostat then the problem would have been greatly alleviated, because instead of the boiler being on for 2 hours and off for 2 hours, it would be on for 5 minutes and off for 5 minutes to maintain the same average hallway temperature, and this would have allowed the living room to maintain its temperature, assuming a 50% boiler duty cycle was sufficient for the heat loss in the living room. (Possibly not in the coldest part of winter, but still a big improvement over the boiler being off for 2 hours at a time)

So in short, TPI is a vast improvement over traditional "off when its too hot and on when its too cold" thermostats.

I'd suggest your real problem is that you have too many radiators without TRV's - ideally you shouldn't have ANY radiators without TRV's except maybe a towel rail. (And even there I like to have my bathroom radiator controlled as well so the bathroom isn't being heated unnecessarily)

You simply can't expect a system with a bunch of radiators around the house with no TRV's to give you good temperature control. I know all too well how poor this is because there were also NO TRV's in our house when we moved in a couple of years ago, with radiators only having manual hand wheels, so the very first thing I had done was to get someone in to install (at the time) conventional manual TRV's, then a year later I installed an Evohome to replace those manual TRV's. Massive improvement with each step.

As for "the boiler never switches off", I think you may still be stuck in an old pre-TPI mindset - that the boiler should switch "off" completely when a room is "up to temperature". What this neglects is that heat loss from the rooms is constant and ongoing, if you simply turn off the boiler completely for an hour or two at a time as conventional thermostats used to, as soon as the radiators cool down the room temperature plummets and then it has to switch on for a long time (an hour or more) to get the rooms up to temperature again.

TPI switches it on and off much more frequently, and will also still switch it on for a certain proportion of time even when the set temperature is reached so that it is maintained rather than letting the temperature drop again. Total gas use is not any more whether your boiler runs in cycles of 5 minutes on 5 minutes off than when it ran 2 hours on 2 hours off, and in fact because you can set your room temperature a bit lower and still maintain comfort you can have a net saving in gas.

So don't labour under the misapprehension that because the boiler is "always on" (it's not always on, its on for a proportion of time every 10 minutes) that it is using more gas. It isn't.

All decent modern digital thermostats (and this includes evohome) use TPI or OpenTherm to control the boiler - there's a good reason for that, it's simply better.